Exemplary embodiments of the invention relate to a device and method for switching electrical load circuits, comprising an electromagnetic contactor having a magnetic drive that is formed from a magnet yoke with a magnet coil and a magnet armature to which a contact bridge is coupled as a movable contactor contact by means of a contact retainer, wherein in the switched-on state, the contactor generates a magnetic retaining force for contacting the contact bridge with fixed contacts, wherein the retaining force results from a magnetic field generated by the magnet coil, and the retaining force is greater than an armature opening force.
German patent document DE 199 47 105 C2 discloses a method and associated arrangements for switching electrical load circuits in which the respective load circuit comprises a contactor having a magnetic drive. The magnetic drive has a magnet yoke with a magnet coil and a magnet armature, to which bridge contacts are coupled as movable contactor contacts by means of a contact retainer, and which, in the switched-on state, generates a magnetic retaining force between the magnet yoke and the magnet armature, which retaining force is greater than an armature opening force. The method provides that as a welding protection for the bridge contacts of the contactor's magnetic drive an emergency switch-off takes place through which the value of the magnetic retaining force, during a time that is short with respect to the typical switch-off time of contactors, is lowered below the value of the armature opening time, for which reason an increase of the magnetic resistance in the iron core of the magnetic drive and a resulting decrease of the magnetic field of the coil is caused by a blocking magnetic field, whereby the contact bridge as contactor main contacts remain permanently open.
Exemplary embodiments of the present invention are directed to a device and a method for switching electrical load circuits, which are improved with respect to the prior art.
A device for switching electrical load circuits comprises an electromagnetic contactor having a magnetic drive formed from a magnet yoke with a magnet coil and a magnet armature to which a contact bridge is coupled as a movable contactor contact by means of a contact retainer. In the switched-on state the contactor generates a magnetic retaining force for contacting the contact bridge with fixed contacts, the retaining force results from a magnetic field generated by the magnet coil, and the retaining force is greater than an armature opening force. According to the invention, an overload contact is integrated in an exciting current circuit in such a manner that when the exciting current circuit is closed and a movement of the contact bridge against the magnetic retaining force occurs, the magnet coil can be short-circuited by closing the overload contact.
By means of the device according to the invention, a risk of overloading the contactor in the event of a fault in the load circuit, for example if the contactor is acted on by a short circuit current load that is a multiple of the rated current of the contactor, is advantageously at least reduced. The overload contact, by means of which the magnet coil can be short-circuited, avoids the occurrence of repeated opening and closing of the contacting between the contact bridge and the fixed contacts, which is referred to in the literature as electromagnetic repulsion, levitation or fluttering, as a result of which the contact bridge is welded to the fixed contacts due to arc formation between the contact bridge and the fixed contacts. Because of this, the contactor remains permanently closed even after turning off an exciting voltage, so that a device to be switched-off is unintentionally still loaded with a supply voltage.
Due to the fact that welding of the contact bridge to the fixed contacts can be largely excluded by means of the device, destruction of the contactor can be avoided and the contactor can be of further use.
Upon a first unintended magnetic lift of the contact bridge from the fixed contacts, the exciting current circuit can be deactivated so that another contacting of the contact bridge with the fixed contacts is avoided.
The overload contact is preferably formed by the contact retainer and a contact element, wherein the contact retainer has a first contact point that is located on a side facing away from the contact bridge, and the contact element has a second contact point, wherein the contact element is spaced apart from the contact retainer when the contact bridge is in contact with the fixed contacts and also in the switched-off state of the exciting current circuit. Due to the fact that the overload contact is formed in this manner, a separate current circuit is formed within the exciting current circuit. The effect of electromagnetic repulsion is always associated with a mechanical movement of the contact retainer together with the contact bridge and the magnet armature, wherein this movement is utilized in a particularly advantageous manner for arranging the contact points. Thus, in the event of a fault in the load circuit, the activation of the magnet coil can be interrupted, as a result of which the contactor is protected to the greatest possible extent against welding of the contact bridge to the fixed contacts.
In an advantageous configuration, the contact element as an integral part of the overload contact can be moved by means of the magnetic field generated by the magnet coil so that advantageously no further element for generating the movement of the contact element for short-circuiting the magnet coil is required.
In another particularly advantageous configuration, a switching unit is arranged between the overload contact and the magnet coil, by means of which switching unit closing of the overload contact after closing of the exciting current circuit and prior to a contacting of the contact bridge with the fixed contacts can be avoided. By means of the switching unit, which, for example, is formed at least by means of a thyristor, it can be avoided that the magnet coil, despite a closed overload contact, cannot be short-circuited. Thus, in first instance, a current flow for short-circuiting the magnet coil is prevented by means of the switching unit.
Preferably, a fuse or a semiconductor switching element, which disconnects an exciting voltage source from the magnet coil after activation of the overload contact, is arranged in the exciting current circuit. By means of the fuse or the semiconductor element it substantially avoids in a particularly advantageous manner the contactor being activated again once the effect of electromagnetic repulsion reoccurs after the contactor is completely switched-off and the exciting voltage is still applied. The fuse or the semiconductor switching element is tripped, as a result of which the exciting current circuit is intentionally interrupted.
In a possible embodiment, the armature opening force can be generated by means of at least one spring element, wherein the spring element can be pretensioned when the magnet coil is energized so that contacting takes place between the contact bridge and the fixed contacts, wherein the retaining force generated by means of the magnetic field, as described above, is greater than the armature opening force resulting from the pretension of the spring element.
The invention further relates to a method for switching electrical load circuits comprising an electromagnetic contactor having a magnetic drive that is formed from a magnet yoke with a magnet coil and an magnet armature to which a contact bridge is coupled as a movable contactor contact by means of a contact retainer, wherein in the switched-on state, a magnetic retaining force for contacting the contact bridge with fixed contacts is generated by means of the contactor, wherein the retaining force results from a magnetic field generated by the magnet coil, and the retaining force is greater than an armature opening force. According to the invention, an overload contact is integrated in an exciting current circuit in such a manner that when the exciting current circuit is closed and a movement of the contact bridge against the magnetic retaining force occurs, the magnet coil is short-circuited by closing the overload contact.
Particularly preferred, a contact element for forming the overload contact is moved by means of the magnetic field generated by the magnetic field.
Parts that correspond to one another are designated in all Figures with the same reference numbers.